A telephone network consists of transmission systems that carry electrical information signals between two or more points; telephone sets that permit subscribers to transmit and receive the signals; and one or more switching systems that temporarily connect one transmission line to another line and, thus link two telephone sets. Since certain types of signals are better suited to certain types of transmission media because of bandwidth requirements, loss limits, etc., the characteristics of the type of signal traffic (e.g., voice, data, and video, in either analog, digital or hybrid form) dictate the physical configuration of the transmission systems used by the network. Consequently, a telephone network may be made up of a variety of transmission systems, each consisting of a transmission medium and its supporting structures, such as, telephone pole lines, underground conduit systems, buried systems, etc. Common transmission media include optical fibers and coaxial cable. The most prevalent type of transmission medium used in present telephone networks, however, is metallic paired cable. Metallic paired cable is composed of copper or aluminum conductors which are coated with an insulated material and which are twisted together into cables of various sizes, depending on the number of paired cables desired. The cables are further contained in a protective sheath that provides environmental protection.
The use of metallic paired cable for signal transmission is accompanied by high attenuation of the signals from one point to another. Attenuation is reduced in transmission systems using metallic paired cable to carry analog signals by the placement of lumped inductances, known as load coils, along the transmission lines. In transmission systems using metallic paired cable to carry digital signals, such as T1 carrier systems, attenuation is eliminated by the use of devices known as repeaters that receive digital signals, amplify or reshape the signals and then retransmit the modified signals. A repeater is commonly constructed of various discrete electrical components, such as integrated circuit chips, diodes, clocks, etc., that are assembled and mounted on a printed circuit board. The repeater printed circuit board is packaged in a casing and installed in the transmission system to connect or terminate a transmission line. As a practical matter, a group or bank of repeaters are usually installed so as to connect into a corresponding bank of transmission lines. In such case, the casing for each individual repeater printed circuit board is not utilized and the printed circuit boards are placed side by side next to one another within a respective printed circuit board rack or cabinet.
Each component in a transmission system using metallic paired cable is at risk when certain fault conditions, such as power cross (i.e., the contacting of a telephone transmission line with an electrical power line), exist on any transmission line or cable. If a fault condition is not isolated from the circuitry of the components in the system, the fault condition may cause damage, such as fire, overheating, mechanical failure, etc. To isolate such fault conditions from the circuitry in a repeater, a thermal cut-off resistor is placed in the current path of the repeater circuitry to interrupt the high current flow resulting from a fault condition. In operation, high fault current passes through a resistive element of the cut-off resistor and the heat generated by the resistive element causes a fuse element to open within the current path. As a result, the high fault current is interrupted and the circuitry of the repeater is isolated and protected from the fault current.
A drawback of existing thermal cut-off resistors, however, is the mechanical design. In particular, existing thermal cut-off resistors are designed and manufactured as a component having a discrete resistor and a separate discrete fuse, both molded into an insulated casing and with one element placed atop the other element therein. This "vertical" design results in a high profile, or height relative to the printed circuit board surface, for the cut-off resistor which becomes a problem in newly designed repeaters that take advantage of the miniaturization of other board components and in new repeater applications that require close circuit board to circuit board spacing. Although the discrete elements can be miniaturized in order to reduce the profile, there is a limit as to how small the elements can become without adversely affecting the functional characteristics of the elements. Another alternative to achieve a lower profile for an existing cut-off resistor is to arrange the two elements in a horizontal rather than a vertical configuration relative to the printed circuit board surface. Disadvantageously, such a modification exacts a trade-off, namely, the horizontal arrangement increases the "footprint" of the cut-off resistor on the circuit board (i.e., the area of board surface taken up by the cut-off resistor mounted thereon) and, thus, fails to conserve valuable board space, which is a standard design goal for printed circuit board components.
The mechanical design of existing thermal cut-off resistors also affects the efficiency of their operation. In particular, the shapes of the resistor and the fuse, and the spatial relationship with one another, causes the heat radiated by the resistor to be dispersed and not specifically directed towards the separate fuse. As a result, the fuse receives only a portion of the radiated heat and responds more slowly to a fault condition. In addition, the fuse does not receive the radiated heat quickly or uniformly along its body. This sluggishness in the heat transfer and heat response of an existing cut-off resistor leads to an imprecise and inefficient operation and can be particularly damaging to the repeater circuitry in the event of a severe or sudden fault condition.
As is evident, the mechanical and operational drawbacks of existing thermal cut-off resistors are not limited to repeaters, but are equally applicable to any device having a printed circuit board and any printed circuit board application.
Consequently, there is a need for a thermal cut-off resistor that has a lower profile than currently available. Further, there is a need to have such a low profile thermal cut-off resistor without increasing the footprint of the cut-off resistor on a printed circuit board. Further, there is a need to have such a low profile thermal cut-off resistor without affecting the functional characteristics. Moreover, there is a need to have a thermal cut-off resistor that has an increased responsiveness to fault conditions. There is also a need to have such a thermal cut-off resistor that can withstand repeated lightning or simulated lightning transients.